The present invention relates to a charged-particle microscope that scans the surface of a sample, more particularly to a scanning charged-particle microscope that enables both focal depth and resolution to be improved at the same time.
The scanning electron microscope (hereinafter referred to as SEM), which is one type of electron microscope, is an observation apparatus that, as disclosed in, for example, Japanese Application Patent Laid-Open Publication No. Hei05-94798, enables the surface shape of a sample to be displayed on an image display unit (for example, a CRT monitor) by focusing through an electromagnetic lens and the like the electron beam emitted from the electron gun inside, then scanning this electron beam in two-dimensional form on the sample, and detecting the secondary charged particles emitted from the sample.
The surface of the sample to be observed using SEM is usually not uniform. For a semiconductor device, for example, humps such as wiring, and recesses such as contact holes, are present in mixed form on the device. To obtain a clear image of such a sample, it is necessary that the diameter, dp, of the electron beam should be equal to or less than image resolution and that such a status should be maintained over the entire observation region. That is to say, the differences in level between the humps and recesses formed in the observation region need to stay within the focal depth, DF, of SEM.
The focal depth, DF, can be approximately represented using the following expression:DF=dp/α  (1)where α is the half-opening angle of the beam.
For SEM, however, it is different to enlarge the focal depth, especially, during high-magnification observation, because, in the case of SEM, the electron beam needs to be focused into a probe shape for improved spatial resolution and the beam diameter, dp, cannot therefore be increased and because a decrease in the beam half-opening angle, a, may affect diffractive aberration.
Furthermore, lamination in a vertical direction in addition to mounting-density enhancement of two-dimensional circuit elements has come to be demanded towards recent semiconductor devices in order to achieve a higher degree of integration. In the case of SEM, which is intended to observe high-density semiconductor devices, although the beam diameter, dp, needs to be reduced for enhancing the spatial resolution of the electron probe during high-magnification observation, since the focal depth is reduced by the relationship of calculation expression (1), the focus of the beam which has been focused on a contact hole deeply formed by lamination deflects at the bottom of the contact hole, with the result that the image of the sample becomes partly indistinct.
In other words, SEM has a contradictory relationship between the spatial resolution and focal depth of its electron probe, and this contradiction poses the problem that SEM cannot be applied to semiconductor devices particularly high in the degree of integration.